The subject of this invention is a fixed plate of hard material for a hydraulic valve, of the type, mainly used in the socalled screw type faucets, which has a passage opening asymmetrically located with respect to the plate center.
A fixed plate of the type to which this invention refers is used by forming a pair with a similar movable plate, and this pair of plates is mounted in the valve between an inlet connection and an outlet connection. Each plate is limited, on a working face, by a lapped planar surface, and the lapped surfaces of both plates are in mutual contact and are suitable for effecting a hydraulic sealing. The movable plate is so mounted that it may be turned with respect to the fixed plate by means of a control member, whereby, when both plates have their passage openings in mutual register, the valve allows passage of a maximum flow, when the passage openings of the two plates are in diametrically opposite positions the flow is shut out, and in the intermediate positions the passage of a partial flow is allowed. The control field of such valve is about 180.degree..
These pairs of plates give rise to a particular problem. Because the passage opening is located in an asymmetrical position, the force applied by the water pressure to the plates is asymmetrical too, and it causes strain concentrations. The hard materials forming the plates are generally rather brittle, and therefore the mutual displacement of the plates under these strain concentrations causes tiny splinterings, from which ensues the formation of little chips of hard material. These chips are confined between the lapped planar surfaces of the two plates forming the pair, from which they have been detached, and cannot easily escape because these surfaces extend on the entire area of the plates, with the only exception of the passage openings. With the lapse of time such chips give rise to grooves in the lapped surfaces, and thus jeopardize the perfect behaviour of the valve.
On the contrary, the described disadvantage does not appear in some plates which are similar to those now considered but have two diametrically opposite passage openings. In this case the forces applied by the water pressure are always symmetric; such symmetric forces do not give rise to strain concentrations, and therefore no splinterings take place and no hard material chip is formed. But the plates showing such a symmetrical disposition of the passage openings have a control field of 90.degree. only, which is too limited for allowing an easy and accurate control of the valve.